At the moment I don't want to spend cash on a new router. I guess OpenBSD is secure enough to connect to the Internet. I say this coz I read many articles saying the same. i double checked my router is not supported by DDWRT.

It is. Many of us use OpenBSD connected directly to the Internet, myself included. The key is in how you configure your OpenBSD system.

If you happen to have a spare system lying around, you can make your own OpenBSD router on the cheap. All you need is two Ethernet ports, an unmanaged switch, and this OpenBSD FAQ page: https://www.openbsd.org/faq/pf/example1.html (and you can ignore the WiFi parts if you don't want WiFi).

I've even done it with a laptop with a dead screen and a USB->Ethernet dongle.

And then you don't have to worry about bloaty impossible to not misconfigure desktop software.